I had an interesting experience with the concept of "passing it forward", but instead of charity, it was griping.
At the gym, I was getting dressed after going for a swim, and Person A walked in and launched into a several minute diatribe about some injustice that he perceived during his workout. Apparently he was walking around the track, and another walker, Person B, asked him to walk in slower lane. He felt that he was going fast enough to merit being on the faster lane. It didn't sound like that big of a deal, but by the time he got to the locker room he was spewing.
He left the locker room, and Persons C and D, who listened to the rant, started griping about how much he was griping. They complained about several minutes about it.
That left me, Person E, thinking about the whole interaction and just shaking my head about it.
So, there you have it... a simple unpleasant interaction rippled through various interactions. There is something to everyday love and courtesy in even the smallest of interactions.